In working reactive metals from metal ingot to finished mill product and after finished part hot working, it is necessary to remove certain surface layer material of metal oxide or, in the case of titanium and titanium alloys, what is commonly referred to as alpha case. These oxygen-enriched phases occur when reactive metals are heated in air or oxygen-containing atmospheres. The oxide layer can affect material strength, fatigue strength, and corrosion resistance of the metal. Titanium and titanium alloys are among the reactive metals, meaning that they react with oxygen and form a brittle tenacious oxide layer (TiO2 for Ti, ZrO2 for Zr, etc.) whenever heated in air or an oxidizing atmosphere above about 480° C. (900° F.), depending on the specific alloy and oxidizing atmosphere. The oxide layer is created by heating of the metal to necessary temperatures for typical mill forging or mill rolling, as a result of welding, or by heating for finished part forging or hot part forming. Reactive metal oxides and alpha case are brittle, and upon forming are routinely accompanied by a series of surface microcracks which penetrate into the bulk metal, potentially causing premature tensile or fatigue failures, and making the surface more susceptible to chemical attack. Therefore, the oxide or alpha case layer must be removed before any subsequent hot or cold working, or final component service.
It is also important when working reactive metals such as titanium and titanium alloys from ingot to finished part, that the cracks formed by thermal and mechanical processing be removed. As described above, these cracks may go deeper than the alpha case and penetrate the bulk metal. Reactive metals are typically heated, hot processed (e.g., forged, rolled, drawn, extruded), cooled, and reheated for additional hot processing between 4 and 8 times to turn an ingot into a finished mill product. The mill product is often again heated for finished part fabrication using techniques including, but not limited to, hot spin forming, ring rolling, superplastic forming, and closed die forming. Each time the metal is cooled after hot processing, cracks form at the surface and extend into the workpiece. In conventional processing, these cracks, are removed by grinding, which involves mechanically removing, or chemical milling in a strong acid, typically HF—HNO3, a uniform thickness layer or amount of material from the workpiece until the bottom of the deepest crack is exposed and removed. Grinding or chemical milling to this depth ensures that all of the cracks are removed, but takes a significant amount of time and labor and also results in a significant and costly loss of material. This is because the cracks sometimes extend into the workpiece to a depth of 5% or more of the thickness or diameter of the workpiece or finished part. But removal of the cracks is necessary, because if the cracks are not removed prior to a subsequent hot working step, or use of a finished part in service, the cracks can propagate and ruin the workpiece or finished part.
In chemistry and manufacturing, electrolysis is a method of using direct electrical current (DC) to drive an otherwise non-spontaneous chemical reaction. Electropolishing is a well known application of electrolysis for deburring metal parts and for producing a bright smooth surface finish. The workpiece to be electropolished is immersed in a bath of electrolyte solution and subjected to a direct electrical current. The workpiece is maintained anodic, with the cathode connection being made to one or more metal conductors surrounding the workpiece in the bath. Electropolishing relies on two opposing reactions which control the process. The first of the reactions is a dissolution reaction during which the metal from the surface of the workpiece passes into solution in the form of ions. Metal is thus removed ion by ion from the surface of the workpiece. The other reaction is an oxidation reaction during which an oxide layer forms on the surface of the workpiece. Buildup of the oxide film limits the progress of the ion removal reaction. This film is thickest over micro depressions and thinnest over micro projections, and because electrical resistance is proportional to the thickness of the oxide film, the fastest rate of metallic dissolution occurs at the micro projections and the slowest rate of metallic dissolution occurs at the micro depressions. Hence, electropolishing selectively removes microscopic high points or “peaks” faster than the rate of attack on the corresponding micro depressions or “valleys.”
Another application of electrolysis is in electrochemical machining processes (ECM). In ECM, a high current (often greater than 40,000 amperes, and applied at current densities often greater than 1.5 million amperes per square meter) is passed between an electrode and a metal workpiece to cause material removal. Electricity is passed through a conductive fluid (electrolyte) from a negatively charged electrode “tool” (cathode) to a conductive workpiece (anode). The cathodic tool is shaped to conform with a desired machining operation and is advanced into the anodic workpiece. A pressurized electrolyte is injected at a set temperature into the area being machined. Material of the workpiece is removed, essentially liquefied, at a rate determined by the tool feed rate into the workpiece. The distance of the gap between the tool and the workpiece varies in the range of 80 to 800 microns (0.003 to 0.030 inches). As electrons cross the gap, material on the workpiece is dissolved and the tool forms the desired shape into the workpiece. The electrolyte fluid carries away metal hydroxide formed in the process from the reaction between the electrolyte and the workpiece. Flushing is necessary because the electrochemical machining process has a low tolerance for metal complexes accumulating in the electrolyte solution. In contrast, processes using electrolyte solutions as disclosed herein remain stable and effective even with high concentrations of titanium in the electrolyte solution.
Electrolyte solutions for metal electropolishing are usually mixtures containing concentrated strong acids (completely dissociated in water) such as mineral acids. Strong acids, as described herein, are generally categorized as those that are stronger in aqueous solution than the hydronium ion (H3O+). Examples of strong acids commonly used in electropolishing are sulfuric acid, hydrochloric acid, perchloric acid, and nitric acid, while examples of weak acids include those in the carboxylic acid group such as formic acid, acetic acid, butyric acid, and citric acid. Organic compounds, such as alcohols, amines, or carboxylic acids are sometimes used in mixtures with strong acids for the purpose of moderating the dissolution etching reaction to avoid excess etching of the workpiece surface. See, for example, U.S. Pat. No. 6,610,194 describing the use of acetic acid as a reaction moderator.
There is an incentive to reduce the use of these strong acids in metal finishing baths, due primarily to the health hazard and cost of waste disposal of the used solution. Citric acid has previously become accepted as a passivation agent for stainless steel pieces by both Department of Defense and ASTM standards. However, while prior studies have shown and quantified the savings from using a commercial citric acid passivation bath solution for passivating stainless steel, they have been unable to find a suitable electrolyte solution in which a significant concentration of citric acid was able to reduce the concentration of strong acids. For example, a publication titled “Citric Acid & Pollution Prevention in Passivation & Electropolishing,” dated 2002, describes several advantages of decreasing the amount of strong mineral acids by the substitution of some amount of a weaker organic acid, and in particular citric acid, due to its low cost, availability, and relatively hazard free disposal, but ultimately evaluated an alternative electrolyte comprising a mixture of mostly phosphoric and sulfuric acid, with a small amount of an organic acid (not citric acid).